In semiconductor device manufacturing, it is necessary to align wafers during the manufacturing process. Semiconductor wafers have well defined shapes and sizes. Standard wafer shapes include centroids having one or more flat sides at their edge. Semiconductor wafers are manufactured with very accurate dimensions. During semiconductor device manufacturing, the wafers are subjected to many chemical processes. Because the different chemical processes are carried out in separate reaction chambers, the wafers have to be transported from one reaction chamber to another. During transportation and handling, the wafers are extremely vulnerable to particulate contamination. Therefore it is advantageous to use a single wafer, multiple chamber, integrated vacuum wafer processing system capable of performing a variety of processes without removal of the wafer from the vacuum enclosure.
In prior art wafer processing systems (such as cluster tool 10 of FIGS. 1A to 1D) the wafers enter via entry cassette 12 and are transported, one at a time, horizontally by a robot arm 14 in a repeatable and accurate manner, first to a wafer aligner 11 in chamber 13A and then from chamber 13A to consecutive chambers (such as 13B, 13C and 13D) and exit via exit cassette 16. The robot arm is capable of some vertical movement in the process chambers to allow the arm to perform a procedure called a handout. During a first handout, a two-pronged terminal portion of the robot arm (called "end effector") is moved underneath one of several wafers stacked in entry cassette 12. Then the cassette moves vertically downward by a distance sufficient for the wafer to be picked up by the robot arm's end effector 15.
As a general rule, wafers in entry cassette 12 do not have a well defined position with respect to the entry cassette. The position of a wafer's center can be shifted and rotated with respect to the position of other wafers' centers and with respect to the center of the entry cassette. Also, a wafer's flat side can be at any random angular orientation. As a result, a wafer's initial position on the robot arm's end effector (prior to any processing) can vary substantially.
FIG. 2 shows a wafer 22 with center O.sub.2 in a position different from desired position 29 (shown with dotted line) with center O.sub.1 that coincides with the end effector's center. Such a shift in the wafer's initial position from a desired position (henceforth "initial" shift) can be quite detrimental to the workings of a cluster tool. Therefore, after a wafer 22 is taken out of entry cassette 12, it is necessary to first transfer wafer 22 to a wafer aligner 11 such as the one shown in chamber 13A of FIG. i and in greater detail in FIG. 2. The wafer's position is corrected and made the same for each wafer in relation to the end effector by wafer aligner 11. After alignment, wafer 22 is transferred to a process chamber (such as chamber 13B of FIG. 1) for further processing. In process chamber 13B, robot arm 14 lowers wafer 22 until wafer 22 rests on a receptor in the chamber. Then the robot arm 14 withdraws into a central chamber 18 (see FIGS. 1A-1D). After the processing is completed, the robot arm 14 returns, picks up the wafer 22 in a handout and moves the wafer 22 to consecutive chambers such as, for example, chambers 13C and 13D (see FIG. 1C) thus subjecting the wafer 22 to several handouts. Each handout creates a likelihood of a shift in a wafer's position from its desired position 29 (henceforth "process" shift) by linear distance ar at an angle .psi. (see, for example, FIG. 2) and a likelihood of the wafer's flat side rotating from the desired angular orientation by an angle .gamma.. In FIG. 2, angle .psi. is measured with respect to the direction of fork movement 25 and angle .gamma. is measured with respect to a radial line passing through the center of rotation of robot arm 14 (FIG. 1). The distance .DELTA.r between the actual position of the wafer's center and the desired position is the eccentricity of the wafer. Over time, as a result of several handouts, the eccentricity and the angular misalignment of a wafer can accumulate.
Detection of a wafer's shape and center is done in wafer aligner 11 by illuminating the top surface of the wafer and rotating the wafer 360.degree. over a linear optical sensor 24 (FIG. 2) capable of instantaneously determining the percentage of the sensor's length covered by the shade of the wafer. The signal output of sensor 24 and the corresponding angular position of the wafer are used by a microprocessor (not shown) to calculate the actual position of the center of the wafer as well as the wafer's angular misalignment. This operation of rotating a wafer and determining its position and misalignment is called scanning.
Centering and angular alignment of wafer 22 are accomplished by wafer aligner 11 in chamber 13A by first rotating wafer 22 in the anticlock wise direction 27 by a calculated angle .psi. (FIG. 2), then lifting wafer 22 from a turntable 20 used to rotate the wafer, translating the wafer 22 horizontally for a calculated distance .DELTA.r using a dedicated fork 26 having vertical and horizontal movement capability and finally lowering the wafer 22 back onto turntable 20. These four steps can be adequate to bring the center of the wafer to the desired position. Finally, the wafer can be rotated by a desired angle such as .psi.-.gamma. so that flat side 28 of wafer 22 is brought into an angular orientation necessary for further processing of the wafer.
The prior art wafer aligner 11 is disadvantageous because during scanning, the wafer 22 is rotated before the wafer's center is made coincident with the center of rotation. Rotation of the wafer 22 with the wafer's center of mass being offset from the axis of rotation results in a centrifugal force on the wafer. In the above arrangement, the wafer 22 is held on the turntable 20 only by frictional forces which are quite small due to smoothness of the wafer. If the centrifugal force overcomes the frictional forces (as is more likely to happen when the rotational speed is increased to increase throughput), the wafer's center will move further away from the center of rotation thus making the wafer's position worse instead of correcting the wafer's position. Therefore an increase in rotational speed to shorten the scanning time for improving throughput is not feasible because of the resulting higher centrifugal force.
Also in wafer aligner 11 described above, the fork 26 is operated through a barrier for maintaining reduced pressure in the chamber 13A. In the prior art, the fork 26 is sealed using vacuum bellows (not shown). However, vacuum bellows create particulate contamination. Also, vacuum bellows restrict the horizontal movement of the fork 26 so that the eccentricity of the wafer 22 may not be correctable in a single step. If an additional step is required to completely correct the wafer's position the resulting throughput is reduced. Also, such an additional step may require a handout thus creating additional particulate contaminants. Finally, because throughput of the prior art wafer aligner 11 is limited, it is not practical to use the wafer aligner 11 to correct process shifts created by handouts in consecutive process chambers (as described above). The inability to correct process shifts imposes a limitation on the number of process chambers and the types of processes that can be performed on a prior art cluster tool.